All the mammalian and avian type C viruses contain a major structural phosphoprotein. The purified phosphoprotein from each virus specifically binds to the homologous viral RNA genome. Direct and competition binding assays have been developed to study the specificity of such protein-RNA interaction in the different classes of type C viruses. The phosphoprotein from each virus can be subfractionated on the basis of the extent of phosphorylation. Such modification has been shown to modulate the extent but not the specificity of its binding to the viral RNA genome. A subpopulation of the viral structural phosphoprotein, characterized by a low level of phosphoprotein, can be specifically recovered from the intact, crosslinked 70S genome of UV irradiated type C viruses. The close association with the viral genome, the ability of specific interaction with homologous viral RNA and the covalent modification (phosphorylation) suggest a possible regulatory role of the type C viral phosphoprotein in host-virus interaction. BIBLIOGRAPHIC REFERENCES: Sen, A. and Todaro, G.J.: Specificity of in vitro binding of primate type C viral RNA and the homologous viral p12 core protein. Science 193: 326-328, 1976. Sen, A. and Todaro, G.J.: The genome-associated, specific RNA binding proteins of avian and mammalian type C viruses. Cell 10: 91-99, 1977.